Direct Current (DC) systems or Grids and DC interconnections are often connected to alternating current (AC) transmission systems. Both AC and DC systems have recently seen important changes. The AC systems are today more and more connected to non-linear sources or generators, such as wind farms. These are of interest to use as sources from an environmental point of view. However, since such sources of energy are used the systems are not stable. This means that in order to increase the stability, they have to be supported. One way of supporting a system is through providing an energy storage system capable of receiving energy from, and to deliver energy to, the AC system. The support can here involve a number of different types of applications, such as transmission enhancement, power oscillation damping, dynamic voltage stability, tie line control, short-term spinning reserve, load leveling, under-frequency load shedding reduction, sub-synchronous resonance damping, and power quality improvement.
One way of providing this energy storage system is through the use of a bank of battery modules. Normally such battery modules are connected to the AC grid via a separate AC/DC converter.
However, as DC systems are becoming more interesting to use in relation to AC systems, it would be of interest to use these DC systems also for supporting an AC system.
It was previously mentioned that there has been important changes also in DC systems. One such change is the introduction of the cell based voltage source converter. This converter, which is for instance described in DE 10103031 is made up of a number of cells, each including a number of switching elements and a cell capacitor. The cells can here be controlled to provide a voltage contribution using the cell capacitors, where the voltage contribution from the various cells forms an AC voltage.
In order to smooth the DC voltage on a DC line these cells can also include a further energy storage means in parallel with the cell capacitor, where the further energy storage means may be a battery. Cells with this type of further energy storage means are briefly mentioned in US 2008/0205093 and US 2008/0252142.
There is in view of what has been described above a need for a DC system that can be used for stabilizing an AC system using an energy storage system comprising battery modules.